Ore grinding circuit

ABSTRACT

The cyclone feed line of an ore grinding circuit is provided with a Venturi-like constriction in a vertical connecting conduit between a centrifugal slurry pump and a cyclone classifier. Slurry density control water is preferably admitted to the cyclone feed line at the downstream side of the Venturi-like constriction.

241/80 X Fahlstrom et 2/1971 Moffat et a]. 8/1971 241/20 June 3, 1975 2,499,347 3/1950 Richard E. McElvain, 2916 Pelham 6/1963 Rd, Madison, Wis. 537l3 3:59 3g June 21, 1973 Primary ExaminerGranville Y. Custer, Jr. PP N04 3729034 Attorney, Agent, or Firm-James E. Nilles 241/79; 241/80; 241/171 7] ABSTRACT int. Cl. B02c 17/00 The cyclone feed line of an ore grinding circuit is pro- 241/33, 38, 41, 46.15, vided with a Venturi-like constriction in a vertical 241/60, 61, 79, 80, 171; 417/151, 182 connecting conduit between a centrifugal slurry pump and a cyclone classifier. Slurry density control water is References Cited preferably admitted to the cyclone feed line at the UNITED STATES PATENTS downstream side of the Venturi-like constriction.

6/1939 417/182 5 Claims, 5 Drawing Figures United States Patent McElvain 1 ORE GRINDING CIRCUIT [76] Inventor:

22 Filed:

[58] Field of Search..............

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13 CLASSIFIERV NEW FEED WATER 3 SHEET NEW FEED FIG. 1

PIIIEIITFFIJUEZLT FIG. 3

HARDNESS CIRCULATING LOAD WATER DENSITY Io 200 M TIME TIME

FIG.4

HARDNESS I I CIRCULATING LOAD WATER DENSITY FEED TIME

TIME

m Tl C E J m R E M W od .X Z x 30 M zmoz m CYCLONE FEE FLOW I RATE I BACKGROUND OF THE INVENTION The invention relates to the mineral processing art and it is concerned more particularly with an ore grinding system wherein an ore feeder, a grinding mill, a sump, a centrifugal pump and a cyclone classifier are connected in series so as to feed the classifier with a continuous flow of slurry by the pump and so as to return the cyclone underflow to the grinding mill.

In an ore grinding system incorporating the mentioned components, one of the most significant process variables is the periodic change of the ore feed characteristics which occurs during the continuous operation of the system. As the characteristics change, as for instance, the hardness of the incoming ore, the circulating load and the level of the pulp in the pump sump become subject to change and consequently, the performance of the entire system is affected. Accordingly, attempts have heretofore been made to provide the system with suitable controls for maintaining a product output which is as nearly as possible uniform under the varying ore feed characteristics. Most critical in this connection are the density and the flow rate of the slurry which is fed by the centrifugal pump into the cyclone classifier. In order to control these slurry properties, it has heretofore been suggested to vary the amount of water which is introduced into the system, and more particularly to do this by maintaining the slurry at a constant level in the sump and by driving the pump at variable speed.

Prior art ore grinding circuits constructed and operated in the above outlined manner have not been entirely satisfactory in certain respects. Difficulties, for instance, have been encountered in keeping the quantity and the quality of their product output uniform under varying characteristics of the incoming ore. Further, the response of the prior art systems to changes of the ore feeder characteristics has been relatively slow so that the time period for readjustment to a stabihzed operation has been objectionably long. In systems using ball mills, some of the balls were apt to pass from the mill into the slurry pump in which case the pump was not only severely damaged, but the entire pump has In some cases exploded and the operating personconsequently xposed to potential serious injury. Wear of the slurry passage from the pump to the classifier was also often objectionably high, and the power consumption disproportionally high.

SUMMARY OF THE INVENTION The present invention provides an improved Or gr nd ng circuit of the type wherein an ore feeder, a grindIng mill, a sump, a centrifugal pump a a Cyclone classifier or separator are connected in series so that the classifier will be fed with a continuous flow of slurry by the pump and so that the cyclone underflow is returned to the grinding mill.

More specifically, the invention provides an improved ore grinding circuit of the mentioned character which avoids the hereinabove described disadvantages and shortcomings of the prior art and which, more partIcularIy, will afford a number of desirable operating characteristics, namely:

a. Maximize feed and production;

b. Minimize wasteful objectional slimes;

c. Permit the operators to control the grind for liberation of the desired minerals;

d. Conserve power;

e. Extend the life of the pump; and

f. Eliminate the plugging of the discharge line with coarse heavy solids.

These and other objects and advantages will become more fully apparent from the following description of a preferred embodiment of the invention in connection with the accompanying drawings.

DRAWINGS FIG. 1 is a diagram of an ore grinding circuit embodying the invention;

FIG. 2 is an enlarged view of a portion of the circuit shown in FIG. 1;

FIG. 3 is a chart illustrating performance characteristics of a conventional ore grinding circuit;

FIG. 4 is a chart similar to FIG. 3 illustrating performance characteristics of an ore grinding circuit embodying the invention; and

FIG. 5 is a diagram illustrating pump and slurry flow relationships in an ore grinding system embodying the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT In the ore grinding system as diagrammatically shown in FIG. 1, new feed is delivered by a conventional feed mechanism 1 to a conventional ball mill 2 which is supplied with the required grinding water as indicated by the arrow 3. The mixture of ground ore and water produced by the mill 2 is discharged into a conventional sump box 4 as indicated by the arrow 6, and part of the required slurry make-up water for the system is introduced into the sump box 4 as indicated by the arrow 7. A centrifugal pump 8 of conventional design draws slurry from the sump box 4 as indicated by the arrow 9 and delivers it into a slurry conduit line 11 as indicated by the arrow 12. From the conduit line II the slurry passes into a conventional cyclone classifier 13 as indicated by the arrow 14. The fine size ore particles and slime, that is, the circuit product is discharged as overflow from the top of the classifier as indicated by the arrow 16 and the coarse product or underflow of the classifier 1 comes out at the bottom and is returned into the ball mill 2, as indicated by the arrow 17.

A flow sensor 18 which may be a conventional magnetic flow meter, and. a density sensor 20 which may be a conventional gamma gauge are connected with the fluid conduit line 11 to monitor the cyclone feed. The sensors 18 and 20 are functionally interrelated with suitable equipment for controlling the ore feed and the water supply for the system as schematically indicated in dotted lines in FIG. 1.

According to the present invention, the slurry 11 which connects the pump 8 with the classifier 13 includes in a vertically extending portion thereof a constricted section near the pump, affording a solids reentry zone wherein the slurry discharged from the pump is under the influence of the pump impeller's centrifugal force and linear velocity as developed within the pump. As shown schematically in FIG. 1 and in greater detail in FIG. 2, a vertically extending portion of the slurry line 11 contains a fitting 19 which is provided with a Venturi-like constriction. Preferably the fitting 19 is also provided with a water inlet stud 21 in communication with the interior of the fitting at the downstream side of the constriction. The pipe 22 which carries the slurry from the fitting 19 to the classifier 13 has a substantially greater cross-sectional area than the normal outlet 23 of the pump 8, the proportion of the cross-sectional areas ranging for instance between 1:1.5 to l:2. The fitting I9 is located in the conduit line 11 at a point more or less closely to the outlet of the pump 8, so that it will provide a solids-re-entry zone at the downstream side of the Venturi constriction wherein the slurry discharged from the pump and any balls which may have passed from the mill 2 into that slurry are under the influence of the pump impellers centrifugal force and linear velocity as developed within the pump.

The tendency of the accelerated solid slurry particles and of the balls, if any, is to move up along one side of the pipe 22 as indicated by the arrow 24 in FIG. 2, and then down along the other side of the pipe toward the pump. One effect of the Venturi constriction is that it counteracts the tendency of the solid slurry particles and balls to rub against the inside surface of the pipe, and consequently the otherwise severe wear of the pipe will be significantly reduced. Another effect of the Venturi constriction is that it prevents re-entry of coarse slurry particles and balls into the pump. This second effect of the Venturi constriction is most important from a practical standpoint because it not only protects the pump against damage by re-entering balls, but because it also eliminates the potential hazard to which operating personnel have heretofore been exposed in conventional ore grinding plants where pumps are apt to explode due to an undue carry-over of balls from the mill.

The inlet 21 of the fitting 19 provides for the introduction of slurry density control water into the conduit line 11 in addition to the slurry make-up water which is introduced into the ball mill 2 and into the sump box 4. Control of the slurry density control water which is introduced into the fitting 19 is critical for the operation of the entire grinding circuit.

The response of a conventional ball mill-cyclone grinding circuit to variations of grindability, particularly hardness variations of the incoming new ore has heretofore been investigated, and a report on these investigations has been published, as for instance in an article Experiences with a Computer Controlled Pilot Scale Grinding Circuit" by Alban J. Lynch and R. L. Wiegel in the October, 1972 issue of the Mining Congress Journal, pp. 49-56. The curves of group A in FIG. 3 show, as reported in said article, the response of a grinding circuit in which the pump is operated at a variable speed, and the curves of group B in FIG. 3 show the response of a grinding circuit in which the pump is operated at a constant speed. Comparing the two groups of curves A and B, it is seen that in each case an increase of ore hardness increases the circulating load and the density of the slurry. In the case of the variable speed pump drive (Group A) the water supply remains constant and the percentage of the coarse product increases moderately. However, in the case of a constant speed pump drive, as shown by the curves in Group B, the water supply is reduced and the percentage of the coarse product increases at an objectionably high rate.

FIG. 4 shows two groups of curves A and C, the curves of Group A, like in FIG. 3, illustrating the response of a conventional ball mill'cyclone grinding circuit to hardness variations of the new ore, and the curves of Group C illustrating such response of a ball mill-cyclone grinding circuit embodying the invention and having a conventional centrifugal pump operating at constant speed. While in system embodying the invention the circulating load by necessity increases as it does in a conventional circuit, the water supply, slurry density and percentage of coarse product differ drastically from a conventional circuit in which the pump is driven at a variable speed and also from a conventional circuit in which the pump is driven at a constant speed. Most significant is the fact that in a system embodying the invention the percentage of coarse particles in the final product is not affected by an increase of hardness of the new ore. Further, as shown by the Feed curve of the Group C in FIG. 4, the feed decreases slightly but is optimized in response to an increase in hardness of the new ore.

FIG. 5 shows the performance characteristic of a centrifugal pump operating at one constant speed, and the demand characteristic of the cyclone feed for optimum performance. The two curves intersect at the point of normal mass flow in the slurry conduit line 11, which is the sum of the pump flow and of the slurry density control water introduced at the fitting 19, the pump flow being determined by the mill discharge and by control of the normal sump level, and the amount of slurry density control water being controlled by suitable valving in response to signal emission from the sensors 18 and 20. If the pump head requirement rises above the value existing at normal operation, the amount of slurry make-up water admitted to the sump box is reduced slightly but additional slurry density control water is injected at the fitting 19. The mass flow in the slurry conduit line is thus maintained at the required value for optimum product output by the cyclone classifier without incurring the adverse effects which have heretofore been experienced by the conventional practice of adding water only to the pump feed sump. Also, the pulp velocity through the pump volute and impeller is maintained at a steady but reduced rate with a consequent significant saving of pump wear.

In actual operation of an ore grinding circuit embodying the invention, it has been observed that with ore having possible 10-50% added power per ton requirement the system can respond within less than three seconds to optimize the feed rate and maintain the desired mass flow-density relationship for the cyclone feed. The rate of new feed (HP/ton) consisting of softer or lower work index ore can be increased to boost the system capacity 10 percent or more while maintaining the desired grind.

I claim:

1. An ore grinding circuit comprising ore feeding means,.ore grinding means, a sump, a centrifugal pump and a cyclone classifier connected in series so as to feed said classifier with a continuous flow of slurry by said pump and return the cyclone underflow to said grinding means, said pump and classifier being interconnected by a conduit line which in a vertically extending portion thereof includes a constricted section near said pump affording a solids re-entry zone wherein the slurry discharged from the pump is under the influence of the pump impellers centrifugal force and linear velocity as developed within the pump.

2. An ore grinding circuit as set forth in claim 1 and further comprising water admitting conduit means operatively associated with said solids re-entry zone.

3. An ore grinding circuit comprising ore feeding means, ore grinding means, a sump, a centrifugal pump and a cyclone classifier connected in series so as to feed said classifier with a continuous flow of slurry by said pump and return the cyclone underflow to said grinding means, a slurry conduit line between said pump and classifier being provided in a vertically extending portion thereof with a Venturi-like constriction near said pump affording a solids re-entry zone wherein the wherein said ore grinding means comprises a ball mill. :0: =0: 

1. An ore grinding circuit comprising ore feeding means, ore grinding means, a sump, a centrifugal pump and a cyclone classifier connected in series so as to feed said classifier with a continuous flow of slurry by said pump and return the cyclone underflow to said grinding means, said pump and classifier being interconnected by a conduit line which in a vertically extending portion thereof includes a constricted section near said pump affording a solids re-entry zone wherein the slurry discharged from the pump is under the influence of the pump impeller''s centrifugal force and linear velocity as developed within the pump.
 1. An ore grinding circuit comprising ore feeding means, ore grinding means, a sump, a centrifugal pump and a cyclone classifier connected in series so as to feed said classifier with a continuous flow of slurry by said pump and return the cyclone underflow to said grinding means, said pump and classifier being interconnected by a conduit line which in a vertically extending portion thereof includes a constricted section near said pump affording a solids re-entry zone wherein the slurry discharged from the pump is under the influence of the pump impeller''s centrifugal force and linear velocity as developed within the pump.
 2. An ore grinding circuit as set forth in claim 1 and further comprising water admitting conduit means operatively associated with said solids re-entry zone.
 3. An ore grinding circuit comprising ore feeding means, ore grinding means, a sump, a centrifugal pump and a cyclone classifier connected in series so as to feed said classifier with a continuous flow of slurry by said pump and return the cyclone underflow to said grinding means, a slurry conduit line between said pump and classifier being provided in a vertically extending portion thereof with a Venturi-like constriction near said pump affording a solids re-entry zone wherein the slurry discharged from the pump is under the influence of the pump impeller''s centrifugal force and linear velocity as developed within the pump; and water conduit means operatively connected with said constriction.
 4. An ore grinding circuit as set forth in claim 3, wherein said water conduit means are connected with said Venturi-like constriction at the downstream side thereof. 